The present invention relates to a fuel injection control system for an internal combustion engine with a direct fuel injection system, and more particularly to the system for controlling quantity of injected fuel when fuel injection timing is changed.
In a direct fuel injection system for the engine, a fuel injector is provided in a combustion chamber and highly pressurized fuel is directly injected into a cylinder of the engine during a compression stroke and then mixture is ignited by a spark plug. The applicant of the present invention has proposed the fuel injection system in U.S. Pat. No. 5,078,107 as follows. At a heavy engine load, fuel injection is advanced so as to provide a homogeneous charge. In an idling state and a light and middle engine load, the fuel injection is retarded to provide a stratified charge. In the engine, when a combustion mode changes, the quantity of the injected fuel is also changed for controlling engine torque.
In such a fuel injection control system as the direct fuel injection engine, the fuel injection pressure is set higher than a maximum pressure in the cylinder. The fuel injection quantity is determined in accordance with the fuel injection pressure and fuel injection pulse width dependent on the engine operating conditions.
In the engine where the fuel injection timing is changed with the change of the combustion mode, the pressure in the cylinder at the fuel injection timing varies accordingly. As a result, the actual quantity of the injected fuel changes with the cylinder pressure even if the pulse width does not change. Therefore, it is necessary to correct the quantity of the injected fuel so as to coincide with a desired value with accuracy.
Japanese Patent Application Laid-open 59-221433 discloses a fuel injection control system where fuel is injected in an intake manifold and a pressure sensor is provided in a combustion chamber for detecting pressure in the combustion chamber. Based on the detected pressure, the quantity of intake air supplied to the combustion chamber is estimated for calculating the quantity of injected fuel. Thus, an air-fuel ratio is properly controlled at supercharging and in a transient state.
However, in the prior art, fuel injection timing is set to a constant value. Therefore, the quantity of the injected fuel can not be corrected by the prior art at the time when the fuel injection timing changes.